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News from ICTP 105 - Features - Inflation
Three pioneering scientists in the field of inflationary theory shared the Dirac Medal of the ICTP in 2002.
Universal Inflation
What exactly happened at the
moment the universe was created some 14 billion years ago? Can
we trace the universe's origins to a huge fireball that followed
on the heels of the Big Bang?
Andrei Linde, a professor of physics at Stanford University, USA,
and recipient of the 2002 Dirac Medal, has been one of the main
architects of an alternative theory of the universe's origins--one
based on inflation.
"The cosmos," he explains, "became exponentially
large in the blink of an eye--an infinitesimal fraction of a second.
Only then did the universe begin to evolve according to the standard
'post-bang' principles of the Big Bang--an endless process marked
by cooling and slow, continual expansion."
Linde adds that he doesn't think "our universe is unique."
In fact, he contends that an infinite number of ever-expanding
universes, similar to the one in which we live, must exist and
that each of these universes, in turn, is capable of spawning
additional universes in a never-ending tale of cosmic proportions.
Linde has emerged as the most vocal proponent of inflationary
theory. But he would be the first to admit that he is not alone
in this effort. Indeed Alan Guth, Victor Weisskopf professor of
physics at the Massachusetts Institute of Technology, and Paul
Steinhardt, Albert Einstein professor of physics at Princeton
University, USA, have also made critical intellectual contributions
to the theory. For this reason, Linde, Guth and Steinhardt shared
the Dirac Medal of the ICTP in 2002.
ICTP director Katepalli Sreenivasan with
2002 Dirac Medal winners Alan Guth, Paul Steinhardt and Andrei
Linde
Inflationary theory--a construct built on the power of intellectual
insight and the elegance of mathematical analyses--has recently
received a boost from observational science, thanks to 'cosmic'
data collected by astronomical satellites and probes.
Linde, now 55, was born in the former Soviet Union, which was
also the birthplace of inflationary theory. Alexei Starobinsky,
a scientist at the Landau Institute of Theoretical Physics in
Moscow, presented the first theory of inflation in 1979. However,
the idea remained largely confined to a small group of Soviet
cosmologists who, while well known in science circles within their
own country, had only limited contact and visibility among their
counterparts in western Europe and the United States.
Meanwhile, Linde received his doctorate degree in physics from
the University of Moscow's Lebedev Physical Institute in 1974,
focussing his dissertation on cosmological phase transitions.
Based on the findings of his dissertation but first fully articulated
in 1976, his notion that phase transitions experienced in a supercooled
vacuum state may be sufficient to transform a cold universe into
a hot one subsequently became a fundamental element of inflationary
cosmology.
In 1981, Alan H. Guth, a postdoctoral student in physics at Cornell
University, USA, devised his own inflationary model, independent
of any knowledge of the work being done in the Soviet Union.
Indeed it was Guth who chose the word 'inflation' to describe
the universe's exponential expansion in an infinitesimal moment
(estimated to be 10 to minus 37 seconds). During this fleeting
time, matter and energy, at first compressed into a space the
size of a proton, filled the void that became the universe--creating
on a cosmic scale the same dynamics that unfolds when an airbag
is released inside an automobile.
Despite its elegance, Guth's theory contained several shortcomings.
Seeking to refine and improve Guth's model, two other youthful
American physicists, Andreas Albrecht and Paul Steinhardt, working
together at the University of Pennsylvania, constructed yet another
model of inflation and, in 1982, Linde began to offer his contributions
to the field. One year after the fall of communism in 1989, Linde
left for the United States, bringing the disparate strands of
inflationary theory closer together.
What accounts for the growing force of inflationary theory within
the larger field of cosmology? Part of its appeal is that the
theory answers several difficult cosmological questions that remain
unanswered by the Big Bang theory and therefore provides a more
coherent framework for understanding how the universe began and
evolved.
For example, why does the universe's broad landscape appear to
be flat when Einstein's theory of relativity indicates that the
universe should be curved? The answer? Inflationary theory holds
that the universe is so large that we can see only a tiny portion
of it. So, from our limited perspective, the universe seems flat
when in reality it is curved.
Similarly, why have we not been able to detect primeval monopoles--particles
with a single magnetic pole that theoretical physicists have concluded
should be present in our universe? Again proponents of inflationary
theory contend that the universe is so large that such particles
may be literally lost in the vastness of space and thus beyond
the 'detection' capabilities of our current instruments.
Finally, what accounts for the universe's homogeneity and isotropy
at a large scale? Again proponents of inflationary theory have
an answer. Inhomogeneous elements that were present before inflation
were eliminated--or, as cosmologists prefer to say, were 'stretched'
or 'ironed'--during the breathtaking fragmentary moment that led
to the universe's creation. At the same time, inflation produced
its own inhomogeneities resulting from small quantum fluctuations
that rushed into the vacuum. Stars and galaxies emerged from these
'matter-and-energy' ripples.
"In the past," says Linde, "cosmologists could
often only discuss their ideas as abstractions or philosophical
musings. Today, however, we are able to probe the outer reaches
of space to prove or disprove our theories. Even the harshest
critics now acknowledge that our analytical description of the
universe's origins is rooted more in science than science fiction.
That's because cosmology's theories can increasingly be tested
experimentally."
"Cosmologists," Linde is quick to add, "can no
longer contend that a theory is right because it's beautiful and
elegant, as sometimes happened in the past. Experimental data
now plays a crucial role in our research, requiring us to prove
that what we say may be true is, in fact, true."
Linde's personal observations about his work are backed by satellite
observations in space. In 1992, for example, the US National Aeronautics
and Space Administration's (NASA) Cosmic Background Explorer (COBE)
satellite took the first 'pictures' of the energy remnants associated
with the early universe--photos in the form of fossilised microwave
radiation casting faint, frozen images lodged in an environment
that existed when the universe was just 200,000 years old and
its temperature decreased to about 3000 degrees Kelvin. Browsing
through COBE's picture album you can gaze at the predicted elemental
ripples of matter and energy from which new galaxies may be spawned
in the future.
In 1999, the Boomerang mission over Antarctica, a scientific balloon
experiment jointly managed by US and Italian space agencies and
universities, enabled scientists to collect even more detailed
data on the universe that seemed to confirm COBE's results. The
data, however, was based only on a small patch of the universe
and more surveys are currently underway to verify the Boomerang
mission's findings.
Then, just a few months ago, NASA's Wilkinson Microwave Anisotropy
Probe (WMAP), launched in 2001, provided a triumphant confirmation
of inflationary theory through its recording of far-away detailed
pictures of the remnants of the early universe that parallel theorists'
concepts of what we might expect to find.
While Linde welcomes these emerging proofs of his theories, he
has recently taken inflationary principles beyond their initial
theoretical boundaries into new intellectual frontiers. He hypothesises
that the entire universe may consist of an infinite number of
inflationary balls that emerge one from another "much like
a 'cosmic tree' that grows exponentially over time generating
new seedlings that ultimately take root on their own."
"What I am proposing," he notes, "is the existence
of a 'fractal-like,' self-reproducing inflationary universe in
which each new universe abides by its own physical laws, creating--or
denying--conditions that can support life as we know it within
our own universe."
"The inflationary paradigm," Linde proudly notes, "is
no longer an obscure segment of the Big Bang theory, which was
the case just 20 years ago. On the contrary, the Big Bang theory
has now become part of the inflationary model."
Inflationary theory's journey from the backstage to the centrestage
of cosmology has made its most 'star' players--Guth, Linde and
Steinhardt--worthy recipients of the Dirac Medal of the ICTP.